The present application under 35 U.S.C. xc2xa7119 claims priority from and incorporates by reference the subject matter in Swiss Patent Application No. 2001 1975/01 filed Oct. 26, 2001.
The invention concerns a method for the calibration of a Wire Bonder.
A Wire Bonder is a machine with which wire connections are made to semiconductor chips after they have been mounted on a substrate. The Wire Bonder has a capillary which is clamped to the tip of a horn. The capillary serves to secure the wire to a connection point on the semiconductor chip and to a connection point on the substrate as well as to guide the wire between the two connection points. On making the wire connection between the connection point on the semiconductor chip and the connection point on the substrate, the end of the wire protruding from the capillary is first melted into a ball. Afterwards, the wire ball is secured to the connection point on the semiconductor chip by means of pressure and ultrasonics. In doing so, ultrasonics are applied to the horn from an ultrasonic transducer. This process is known as ball bonding. The wire is then pulled through to the required length, formed into a wire loop and welded to the connection point on the substrate. This last process is known as wedge bonding. After securing the wire to the connection point on the substrate, the wire is torn off and the next bond cycle can begin.
The ball bonding can be influenced by various factors. In order to achieve bond connections of a predetermined quality, the optimum values of several physical and/or technical parameters must be determined for a particular process. Examples of such parameters are the bond force, that is the force which, during the bonding process, is exerted by the capillary on the ball or the connection point on the semiconductor chip, or the amplitude of the alternating current with which the ultrasonic transducer applies ultrasonics to the horn.
During bonding, the capillary wears out so that, from time to time, it must be replaced by a new capillary. When the capillary is exchanged, the oscillating behaviour of the tip of the capillary generally changes because each capillary has somewhat different characteristics and is also clamped differently in the horn. Therefore, after each capillary change, specific parameters of the Wire Bonder must be recalibrated, in particular, the amplitude of the alternating current which is applied to the horn by the ultrasonic transducer must be adapted to the new conditions.
A method for calibrating the oscillation amplitude of the horn is known from the U.S. Pat. No. 5,199,630. With this method, the amplitude of the tip of the horn oscillating freely in the air is measured and the amplitude of the alternating current applied to the horn by the ultrasonic transducer is redefined so that the tip of the horn oscillating freely in the air assumes a predetermined value.
A method of calibrating the parameters of a Wire Bonder with which the tip of the capillary or the bond ball guided by the tip of the capillary touches a connection point on a test chip, is known from the European patent application EP 953 398. The test chip contains a sensor integrated in the area of the connection point the signal of which is used for the calibration. A sensor measuring the temperature for example is foreseen for the calibration of the ultrasonic power.
A piezoresistive sensor which enables the registration of the mechanical stresses which occur in silicon during bonding is known from the article xe2x80x9cAnalysis of ultrasonic wire bonding by in-situ piezoresistive microsensorsxe2x80x9d, which was published in the proceedings of the conference on xe2x80x9cTransducers ""01 Eurosensors XVxe2x80x9d which took place in Munich from Jun. 10-14, 2001.
The object of the invention is to develop a method for the calibration of a Wire Bonder which guarantees that, in mass production, semiconductor chips are wire bonded under the same process conditions both before and after a capillary change.
A further object which presents itself in mass-production is the transmission of the optimum parameters found on one Wire Bonder to another Wire Bonder. The invention should also offer a solution for this object and support the transfer of recipes from Wire Bonder to Wire Bonder in a simple and robust manner.
The named objects are solved in accordance with the invention by means of a method with which firstly a reference value is determined for a first Wire Bonder by means of a sensor integrated into a semiconductor chip and with which the same sensor or a sensor of the same type is used to recalibrate the first Wire Bonder for example after capillary exchange or to calibrate another Wire Bonder by means of referring to this reference value.
Every Wire Bonder has a capillary clamped to a horn. Ultrasonics is applied to the horn by an ultrasonic transducer whereby the ultrasonic transducer is controlled by means of a parameter P. For the calibration of the parameter P, a reference value URef is determined in accordance with the following steps:
a1) Placing the capillary without wire or wire ball onto a contact area of the sensor,
b1) Application of a bond force FC to the capillary which is great enough so that, in the next step, the capillary does not slide back and forth on the surface of the sensor 1,
c1) Application of a predetermined value P1 of the parameter to the ultrasonic transducer and acquiring the value U1 of the amplitude of the sensor signal after a stationary condition has been achieved, and
d1) Storage of value U1 of the sensor signal as reference value URef.
The first Wire Bonder will then be recalibrated, for example after a capillary change, or another Wire Bonder will be calibrated in that a correction factor xcex3 is determined in accordance with the following steps:
a2) Placing the capillary without wire or wire ball onto the contact area of the same sensor or another sensor of the same type,
b2) Applying the same bond force FC to the capillary, and
c2) Operating the ultrasonic transducer under the control of the parameter P, and
d2) Determining the correction factor xcex3 so that the amplitude of the sensor signal assumes the value URef when the ultrasonic transducer is operated with the value P2=xcex3*P1.
If, therefore, the ultrasonic transducer of the Wire Bonder calibrated in this way is operated in production with the value P2=xcex3*P1 of parameter P, it is then ensured that the semiconductor chips are wire-connected under the same physical process conditions as the semiconductor chips of the first Wire Bonder immediately after determination of the reference value URef.
In accordance with a preferred embodiment, in step c2), value P1 of the parameter is applied to the ultrasonic transducer and the value U of the amplitude of the sensor signal is acquired after the stationary condition has been achieved and, in step d2), the correction factor xcex3 is determined as xcex3=URef/U.
In accordance with another embodiment, in step c2), the value of parameter P is changed until the amplitude of the sensor signal assumes the value URef whereby the value of parameter P in this condition is designated as P2. The correction factor xcex3 is then determined as xcex3=P2/P1.
The parameter P which is calibrated in this way is, for example, the current flowing through the ultrasonic transducer or the voltage applied to the ultrasonic transducer.
In the following, embodiments of the invention are described in more detail based on the drawing.